1. Field
The presently disclosed subject matter relates to an optical deflector used in a projector, a headlamp and the like.
2. Description of the Related Art
FIG. 1A is a perspective view illustrating a prior art one-dimensional optical deflector, and FIG. 1B is a partial enlargement of the optical deflector of FIG. 1A enclosed by a dotted line B in FIG. 1A (see: FIG. 5 of JP2008-20701A).
As illustrated in FIGS. 1A and 1B, the prior art one-dimensional optical deflector is constructed by a circular mirror 1, a movable ring-shaped frame 2 surrounding the mirror 1 with a slit, a pair of torsion bars 3a and 3b arranged along a Y-axis having ends coupled to the inner circumference of the movable ring-shaped frame 2 and other ends coupled to the circumference of the mirror 1, a support body 4 surrounding the movable ring-shaped frame 2 with a slit, and a pair of coupling bars 5a and 5b arranged along an X-axis perpendicular to the Y-axis having ends coupled to the inner circumference of the support body 4 and other ends coupled to the outer circumference of the movable ring-shaped frame 2.
Piezoelectric actuators (not shown) formed on the entire movable ring-shaped frame 2 associated with the torsion bars 3a and 3b are of a resonance type. That is, in a resonance state, when the rocking frequency “f” of the piezoelectric actuators is close to the natural frequency of a mechanically-vibrating system of the mirror 1, the deflection angle of the mirror 1 with respect to the Y-axis can be increased.
In the above-mentioned resonance state, the inventor found that, portions of the movable ring-shaped frame 2 where the torsion bars 3a and 3b and the coupling bars 5a and 5b are coupled form loops having maximum amplitudes of a resonant vibration, while portions of the movable ring-shaped frame 2 having 45°-angled diameter directions with respect to a diameter line between the torsion bars 3a and 3b and a diameter line between the coupling bars 5a and 5b form nodes having essentially zero amplitudes of the resonant vibration. Thus, the loop portions of the movable ring-shaped frame 2 where the coupling bars 5a and 5b are located would degrade the endurance of the coupling bars 5a and 5b. 
Particularly, in order to suppress the deformation of the mirror 1, a reinforcement would be attached to the back surface of the mirror 1, which would substantially increase the inertial moment thereof. In this case, the endurance of the coupling bars 5a and 5b would further be degraded.
The loop portions and node portions of the movable ring-shaped frame 2 are discussed in more detail below.
In FIG. 1B, radial axes C0, C1, . . . , C8, . . . , C15 are defined at intervals 22.5° centered at a point “0”. Also, a circumferential line L is defined at a center line between the outer and inner circumferences of the movable ring-shaped frame 2. Further, P0, P1, . . . , P8, . . . , P15 are defined as locations at intersections between the circumferential line L and the radial axes C0, C1, . . . , C8, . . . , C15, respectively.
The X-axis is defined as the direction of the radial axis C8, and the Y-axis is defined as the direction of the radial axis C4. In this case, the Y-axis is shifted from the rocking direction of the mirror 1 by a half thickness of the mirror 1; however, since this half thickness is very thin, the Y-axis is substantially the same as the rocking direction of the mirror 1. Also, a Z-axis is defined as a direction perpendicular to the X-axis and the Y-axis.
In FIG. 2, which illustrates the amplitudes at the locations P4, P5, P6, P7 and P8 along the Z-axis of FIG. 1B in a resonant state, three or four amplitudes at three or four X-coordinate values and at one Y-coordinate value were measured. As illustrated in FIG. 2, the amplitude at the location P4 was about 4.4 mm, the amplitude at the location P5 was about 1.6 mm, the amplitude at the location P6 was about 0.3 mm, the amplitude at the location P7 was about 1.5 mm, and the amplitude at the location P8 was about 2.2 mm. Therefore, the amplitude at the location P6 was minimum, while the amplitude at the location P4 was maximum. Also, the amplitudes at the locations P5 and P7 were medium.
The amplitude at the location P8 is smaller than the amplitude at the location P4, because the coupling bar 5b is located at the location P8 to suppress the vibration of the portion of the movable ring-shaped frame 2 at the location P8. That is, if no coupling bar is present at the location P8, the amplitude at the location P8 is considered to be the same as the amplitude at the location P4, i.e., larger than 2.2 mm.
As is understood from FIG. 2, the amplitudes at the locations P0, P1, . . . , P8, . . . , P15 of the circumferential line L in a resonant state can be as shown in FIG. 3. Thus, while the locations P2 and P6 are at nodes of a resonant vibration, the locations P0 and P8 of the coupling bars 5a and 5b, respectively, are at loops of the resonant vibration, which would degrade the endurance of the coupling bars 5a and 5b. 